Abstract
A series of absolute cavity radiometers, designed to measure solar irradiance, has been developed at the Jet Propulsion Laboratory. Analysis indicates the absolute uncertainty of irradiance measurements made by the most recent devices to be less than ±0·5 mW/cm 2. In a series of experiments the radiometric scale defined by the JPL instruments has been compared with the International Pyrheliometric Scale (IPS). A systematic 2·2 per cent difference has been observed with the IPS producing consistently lower measurements. The solar constant and spectral distribution have been derived from high-altitude balloon flight measurements made by two types of JPL cavity radiometers. Measurements at 25 km in 1968 produced a solar constant value of 137·0 mW/cm 2. From 1969 measurements at 36 km, a value of 136·6 mW/cm 2 was derived, with an estimated absolute uncertainty of ±0·5 per cent. The solar spectrum information from these experiments agrees most closely with the solar spectrum model of Labs and Neckel.
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